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		<title>Gravity</title>
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		<h3>Adding Gravity</h3>
		<p>Gravity is a downward force exerted on objects.  Newton's Law
		says that force is equal to mass times acceleration, where
		acceleration is the amount that the velocity of an object is changed
		per second.
		</p>
		<p>Since the force is downwards, and it's equal to mass times
		acceleration, this means the acceleration will be downwards too.
		If you drop a ball, for example, you'll notice that the ball's velocity
		starts to increase in the downward direction.  After all, you just
		released the ball from your fingers, giving it zero initial velocity.
		Why did it start moving?  Because gravity gives it velocity in the
		downward direction.
		</p>
		<p>So...how can we add gravity to our particles?  Just increase the
		velocity in the downward direction, by adding some number to the
		velocity every frame.  The higher the number, the faster the
		particles fall.
		</p>
		<p>NOTE: If your program doesn't work, or doesn't even start, look at
		the Terminal window (a white or black box behind GameJockey).
		If that Terminal window has an error message in it, read the message
		and fix the error!
		</p>
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